1. Field of the Invention
The present invention relates to an optical amplifier for amplifying a signal light while suppressing an optical surge in an optical communication system, and in particular, to an optical amplifier for suppressing the optical surge utilizing an up-conversion phenomenon in an optical amplifying medium.
2. Description of the Related Art
In recent years, a demand for communications traffic is drastically increased with the development of multimedia networks, and an optical transmission system for multi-repeatedly amplifying a signal light using optical amplifiers, has a large role for achieving the economization of communication system in the multimedia society. An optical amplifier is a key device taking a role in the large-capacity and long-distance of the optical transmission system. However, if the input power to the optical amplifier is dynamically changed, there occurs a transient output peak due to gain saturation characteristics to the input power of the optical amplifying medium. As shown in FIG. 12 for example, in a metropolitan area network system or the like in which the number of signal wavelengths (the number of channels) contained in a wavelength division multiplexed (WDM) light is dynamically changed, in the case where the 1st ch. to the 39th ch. are dropped by an optical add/drop multiplexer (OADM) node which has added the 40th ch. to the WDM light containing the 1st ch. to the 39th ch. resulting in that the number of channels in the WDM light to be input to the optical amplifier arranged on the output side of the OADM node is changed to 1 channel from 40 channels, the optical amplifier becomes in a high gain state with a decrease of the total input power, to generate the optical output power of higher level compared with a stationary output power level at the operation time of 40 channels. (the middle stage of FIG. 12.). Such a spike optical output power variation is generally called an optical surge, and an occurrence amount of the optical surge corresponds to a gain difference ΔA[dB]. Further, the optical surge occurred in the optical amplifier is accumulated after passing through a downstream repeating amplifier, to be further increased. As a result, an input signal level to a light receiver at a terminal station problematically exceeds a received light level upper limit value (the lower stage of FIG. 12), leading the degradation of received Q-value.
As a conventional technology for coping with such a problem, there has been known a method of controlling the pumping light power at a high speed (refer to Cechan Tian et al., “Analysis and Control of Transient Dynamics of EDFA Pumped by 1480- and 980 mm Lasers” JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 21, NO. 8, AUGUST 2003) or a method of clamping a gain (refer to Japanese Unexamined Patent Publication No. 10-200182).
To be specific, for the high speed control method of the pumping light power, there has been reported a method of adding a feedforward control to a typical feedback control of the pumping light power to increase a control speed, thereby suppressing the optical surge. For the gain clamping method, there has been proposed a method of discharging the pumping light energy due to the high gain state as the oscillation energy by providing an optical circuit configuration in which a gain or the reflectance of a specific wavelength other than a signal light is increased. Furthermore, there has also been reported a method of suppressing the high gain state by inputting a saturation signal of specific wavelength other than a signal light to an optical amplifier based on a monitoring result of the input power.
As described in the above, to the subject of the suppression of optical surge occurred in the optical amplifier, in the conventional technology, measures have been attempted from the view points of both of the control circuit configuration and the optical circuit configuration.
However, the conventional optical surge suppressing technology as described above has following problems. In the high speed control method of the pumping light power, there is a problem of residual optical surge in that the optical surge occurring at a high speed of change is difficult to be suppressed, depending on the way of giving a control target value and a control circuit constant to arbitrarily changing operating conditions (for example, the number of signal wavelengths, the input power, the gain and the like). Further, in the gain clamping method, since the complexity of optical circuit configuration cannot be avoided, there is a problem of cost escalation or the like of the optical amplifier.